It is a well-known fact that urea, because it contains approximately 46% by weight nitrogen, is potentially one of the best sources of nitrogen for fertilizing soils to stimulate plant growth. However, effective use of urea on crops requiring surface fertilization applications has been limited due to high ammonia losses. These losses have been attributed to the influence of the enzyme urease, which in the presence of moisture appears to cause the hydrolysis of urea according to the following reaction: ##STR1## It is now generally accepted, however, that the urea first reacts with the moisture to form ammonium carbonate, as follows: ##STR2## Ammonium carbonate is a relatively unstable compound, especially in alkaline environments. The compound decomposes according to the following reactions. EQU (NH.sub.4).sub.2 CO.sub.3 +H.sub.2 O.revreaction.2NH.sub.4 OH+CO.sub.2 .uparw. (3) EQU 2NH.sub.4 OH.revreaction.H.sub.2 O+2NH.sub.3 .uparw. (4)
Since ammonia and carbon dioxide are both gases, they readily volatilize into the atmosphere, thus urging the reactions to continue in the direction of decomposition.
It has also been generally recognized that urea applied to a soil surface does not produce serious ammonia loss in the first two days unless soil temperatures exceed 90.degree. F. It is the third and fourth day when the ammonia loss becomes extremely severe. This is due to the fact that the microorganisms that produce the urease enzyme which hydrolyzes the urea must build up, and it generally takes about two days to produce the enzyme. This is because the enzyme is generally produced only when the material it works on, that is, urea, becomes available. When the soil microorganisms have built up sufficient enzyme, the urea is converted to ammonium carbonate according to equation (2). Further breakdown through volatilization of NH.sub.3 and CO.sub.2 according to equations (3) and (4), is usually inevitable.
Heretofore, the only practical solution to this ammonia volatilization loss was thought to be the rapid soil incorporation of urea-containing compositions so that the decomposition products would be exchanged or otherwise absorbed into the soil. Thus, urea is presently recommended for use as a fertilizer only where it can be "banded" into the soil, that is, applied beneath the soil's surface, or where it can be used with crops that are tilled, so that substantially all the urea is immediately placed beneath the soil's surface. The use of urea is generally not recommended on non-row crops, such as pastures, orchards, timber forests and the like, due to the fact that these crops can only be treated by direct application to the soil surface, as by pellets, solution, aerial spraying, or the like. The rapid volatilization of ammonia from surface-applied urea renders it much less effective as a fertilizer.
It has been suggested by a number of investigators that ammonia volatilization loss from urea fertilizers may be reduced by combining the urea with acid generating compounds. This would tend to produce a lower pH environment around the urea, which in turn would tend to prevent the decomposition of the ammonium carbonate formed upon urea hydrolysis. Young, in U.S. Pat. No. 4,073,633, suggests further that the soil pH on a gross basis need not be changed (indeed, this is quite impractical in many soils), only that of the environment immediately surrounding the urea particles. The patent teaches that "it is essential only that the pH in the immediate environment of the particles be maintained at a neutral or slightly acidic level". Col. 1, lines 59-62. At Col. 2, lines 17-33 there are described a broad list of "acid generators", which are stated to be suitable for combination with urea to reduce surface volatility loss.
Further investigation has proven, however, that the problem of ammonia volatilization loss from surface application of urea to soils is not resolved as simply as the Young patent disclosure would suggest. It appears that the system described by Young retards ammonia loss for a relatively short period, but provides little value where the urea is left on the soil surface for more than two days. Moreover, it also appears that the system works effectively only on soils having a high natural pH. In addition, it appears that many of the "acid-generator" compounds specified in the Young patent are not at all effective in reducing overall ammonia loss. In short, prior to the present invention, the art has remained uncertain as to how to effectively inhibit ammonia volatilization losses from surface applied urea. Despite the obvious value of a fertilizer which could provide 46% by weight nitrogen without any unwanted or unnecessary mineral values, urea continues to be recommended only where it can be promptly directed to beneath the soil's surface.